Butanol is an important industrial chemical with a variety of applications including use as a fuel additive, as a feedstock chemical in the plastics industry, and as a food-grade extractant in the food and flavor industry. Accordingly, there is a high demand for butanol as well as for efficient and environmentally friendly production methods.
Production of butanol utilizing fermentation by microorganisms is one environmentally friendly production method. Some microorganisms that produce butanol in high yields also have low butanol toxicity thresholds, such that butanol needs to be removed from the fermentation vessel as it is being produced. In situ product removal (ISPR) (also referred to as extractive fermentation) removes butanol from the fermentation vessel as it is produced, thereby allowing the microorganism to produce butanol at high yields. One method for ISPR that has been described in the art is liquid-liquid extraction (U.S. Patent Application Publication No. 2009/0305370). In order to be technically and economically viable, ideally, liquid-liquid extraction calls for good contact between the extractant and the fermentation broth for efficient mass transfer of the product alcohol into the extractant; good phase separation of the extractant from the fermentation broth (during and after fermentation); efficient recovery and recycle of the extractant; minimal degradation of the capacity of the extractant to extract the product alcohol (by, e.g., preventing the lowering of the partition coefficient for the product alcohol into the extractant) and contamination of the extractant by lipids that lower the partition coefficient over a long-term operation.
In particular, the extractant can become contaminated with lipid over time with each recycle, for example, by the build-up of lipids present in the biomass that is fed to the fermentation vessel as feedstock of hydrolysable starch. As an example, a liquified corn mash loaded to a fermentation vessel at 30 wt % dry corn solids can result in a fermentation broth that contains about 1.2 wt % corn oil during conversion of glucose to butanol by simultaneous saccharification and fermentation (SSF) (with saccharification of the liquified mash occurring during fermentation by the addition of glucoamylase to produce glucose). The dissolution of the corn oil lipids in oleyl alcohol (OA) serving as an extractant during ISPR can result in build-up of lipid concentration with each OA recycle, decreasing the partition coefficient for the product alcohol in OA as the lipid concentration in OA increases with each recycle of OA.
In addition, the presence of the undissolved solids during extractive fermentation can negatively affect the efficiency of the alcohol production. For example, the presence of the undissolved solids may lower the mass transfer coefficient inside the fermentation vessel, impede phase separation in the fermentation vessel, result in the accumulation of corn oil from the undissolved solids in the extractant leading to reduced extraction efficiency over time, increase the loss of solvent because it becomes trapped in solids ultimately removed as Dried Distillers' Grains with Solubles (DDGS), slow the disengagement of extractant drops from the fermentation broth, and/or result in a lower fermentation vessel volume efficiency.
Several approaches for reducing the degradation of the partition coefficient of the extractant used in extractive fermentation have included wet milling, fractionation, and removal of solids. Wet milling is an expensive, multi-step process that separates a biomass (e.g., corn) into all of its key components (germ, pericarp fiber, starch, and gluten) in order to capture value from each co-product separately. This process gives a purified starch stream; however, it is costly and includes the separation of the biomass into its non-starch components, which is unnecessary for fermentative alcohol production. Fractionation removes fiber and germ, which contains a majority of the lipids present in ground whole corn, resulting in corn that has a higher starch (endosperm) content. Dry fractionation does not separate the germ and fiber, and therefore, it is less expensive than wet milling. However, fractionation does not remove the entirety of the fiber or germ, and does not result in total elimination of solids. Furthermore, there is some loss of starch in fractionation. Wet milling of corn is more expensive than dry fractionation, but dry fractionation is more expensive than dry grinding of unfractionated corn. Removal of solids, including germ containing lipids, from liquified mash prior to use in fermentation can substantially eliminate undissolved solids, as described for example in co-pending, commonly owned U.S. Provisional Patent Application No. 61/356,290, filed Jun. 18, 2010. However, it would be advantageous if the degradation of the partition coefficient of the extractant can be reduced even without fractionation or removal of undissolved solids. Thus, there is a continuing need to develop more efficient methods and systems for producing product alcohols such as butanol, through extractive fermentation in which the degradation of the partition coefficient of the extractant is reduced.
Moreover, the extractant (e.g., oleyl alcohol) is typically added to the process, rather than produced at a step in the process and therefore, the extractant is a raw material expense. Since the extractant can be lost to adsorption on non-fermentable solids and diluted with lipids introduced into the process, the economy of the alcohol production process can be affected by the efficiency of the extractant recovery and recycle. Thus, there exists a continuing need for alternative extractants for ISPR that can result in a more economical process by reducing capital and/or operating costs.